/Engine/Source/Runtime/Core/Public/Math/TransformCalculus2D.h
If you look at how the vector multiply works out: [X(AC-BD)+Y(-BC-AD) X(AD+BC)+Y(-BD+AC)] you can see it follows the same form of the imaginary form. Indeed, check out how the matrix nicely works out to [ A B] for a visual proof of the results. [-B A]
[TQuat2](API\Runtime\Core\Math\TQuat2) Concatenate ( const [TQuat2](API\Runtime\Core\Math\TQuat2) & RHS ) const